1. Field of the Invention
The present invention relates to a laser diode module including a light-detecting device, such as a photodiode (PD), for monitoring an optical output from a laser diode device, for example, from a semiconductor laser diode.
2. Description of the Background Art
FIG. 10 shows a structure of a laser diode module including a photodiode. Referring to FIG. 10, in general, the laser diode module includes a laser diode (LD) device 1, a photodiode 2, a lens 4, a cover glass 10, a polarizer 11 and a filter 12. An optical fiber having its end 5 is shown in FIG. 10.
In the conventional laser diode module shown in FIG. 10, photodiode 2 is placed on an optical axis 6 at the back of laser diode device 1. Photodiode 2 thus receives back radiation 7 from the rear side of laser diode device 1 to monitor an output of laser diode 1.
A pumping laser (wavelength: 980 or 1480 nm) employed as a light source for exciting a fiber amplifier has a high optical output. It could accordingly occur that the current monitored by photodiode 2 reaches a saturation state or that the optical output exceeds the driving ability of a driver for photodiode 2.
With the purpose of more accurately monitoring the laser output, it is desired that photodiode 2 receives, not the back radiation 7 which is affected by an internal state (lasing state) of the laser module, but reflected radiation 9 of front radiation 8 or scattered radiation from laser diode device 1.
A laser resonator is constituted of two opposing end-surfaces of laser diode device 1 and a reflector (e.g., a fiber grating within the optical fiber). If any environmental change or change of an injection current is caused to the laser resonator (composite resonator including the fiber grating), gain characteristics of laser diode device 1 change and accordingly the fiber grating does not effectively function, and the ratio between an output from the front and an output from the back of laser diode device 1 changes.
A resultant problem in the placement of photodiode 2 receiving back radiation 7 only is that the linear relation between the radiation received by photodiode 2 and the laser output is lost and consequently photodiode 2 does not serve as an output monitor.
In addition, reflected radiation 9 of front radiation 8 alone is insufficient to allow photodiode 2 to have a sufficient signal intensity, resulting in deterioration of the S/N ratio and thus deterioration of controllability of photodiode 2.